It is known to use resiliently compressible foamed plastics material for wound dressings. This material can permit comfortable application of pressure even on curved or other non-planar body surfaces. Also the material can have good absorption properties suited to use with moist wounds.
It is important or desirable to have surface properties which for the wound-facing inner surface permit easy flow of moisture into the dressing whilst avoiding sticking to the wound, and for the outer surface provide at least to a certain extent, a waterproof breathable barrier. However problems arise in connection with the provision of satisfactory surface properties in the context of foam material. In particular, modification of the surfaces of the foam material, by treatment thereof, or by application of surface layers thereto, tends to give rise to distortion of the foam material as the foam absorbs moisture and swells. Distortion is disadvantageous because it disrupts even application of pressure and can disturb healing of the wound. Also, air pockets may be formed which provide sites for bacterial growth.
British Patent 1417962 describes the use of a non-reticulated polyurethane foam which is modified at the wound-facing inner surface, by application of heat and pressure, to give a layer of collapsed cells, which layer is soft, pliant and facilitates flow of moisture from the wound into the body of the foam material.
A development of this known structure, involving the use of a body of open-celled hydrophilic foam is described in WO 92/13576. This construction facilitates flow of moisture at a relatively high rate suited to use with very moist wounds. However, considerable swelling and consequent distortion can arise.
An object of the present invention is to provide a wound dressing which incorporates an absorbent foam layer and has good surface properties yet which has good dimensional stability and a reduced tendency to distort on absorption of moisture.